Authenticated Fast Handoff

1. November 2001 Tim Moore, Bernard Aboba/Microsoft Authenticated Fast Handoff IEEE 802.11 Tgi Tim Moore Bernard Aboba Microsoft

2. November 2001 Tim Moore, Bernard Aboba/Microsoft

3. November 2001 Tim Moore, Bernard Aboba/Microsoft Fast Handoff Scenarios Enterprise 802.11 wireless access within a corporate campus VLANs may be implemented Authentication may involve passwords, smartcards, token cards, OTP, etc. User authenticates to an authentication server on the same campus “Hot Spot” 802.11 wireless access in airports, hotels, cafes Authentication is typically password-based Account at wireless ISP Wholesale wireless access to corporations may eventually become popular VLANs typically not implemented User authenticates to the home authentication server, which may be far away

4. November 2001 Tim Moore, Bernard Aboba/Microsoft Goals for Authenticated Fast Handoff Fast Transfer times on order of 20 ms or less, not seconds No need to reauthenticate after each reassociation Support for complete context transfer (including VLANID) for seamless user experience Secure Support for per-session keys, dynamic key generation Works with all EAP authentication methods Secure reassociation requests and responses, as well as disassociation notifications Protection against spoofing, denial of service, hijacking Deployable Enable deployment of inter-access point protocol (IAPP) without a registration service

5. November 2001 Tim Moore, Bernard Aboba/Microsoft Security improvements

6. November 2001 Tim Moore, Bernard Aboba/Microsoft

7. November 2001 Tim Moore, Bernard Aboba/Microsoft 802.11 “Classic”: Implications for Fast Handoff Classic 802.11 authentication occurs before reassociation Enables a STA to pre-authenticate with the new AP prior to reassociation Inter-Access Point communication typically not necessary If all APs use the same key, new AP can validate the STA authentication without contacting the old AP. Ability for STAs to quickly reassociate between access points STA sends Disassociate to old AP after it receives Reassociation-Response from new AP New AP install STA state in DS after receiving an ACK of the Reassociation-Response from STA No cryptographic operations in the critical path Management frames are not authenticated Association-Request/Response, Reassociation-Request/Response, Disassociation notification are unauthenticated Enables an attacker to forge these and other management frames, take over sessions

8. November 2001 Tim Moore, Bernard Aboba/Microsoft 802.11 “Classic” Fast Handoff

9. November 2001 Tim Moore, Bernard Aboba/Microsoft

10. November 2001 Tim Moore, Bernard Aboba/Microsoft 802.11i: Implications for Fast Handoff With 802.1X, upper layer authentication occurs after ESN association/reassociation 802.1X state machine is driven by association/reassociation events AP can only be associated with a single STA; since 802.1X authentication occurs after reassociation, an ESN STA can only authenticate to a single ESN AP Full reauthentication to each AP a significant cost 802.1X authentication may involve multiple round-trips, public key operations Environments with many mobile stations can heavily load the backend authentication server Desirable to avoid a full reauthentication at every AP Need to lock all doors left open by classic 802.11 802.11i adds dynamic keying (802.1X), credible ciphersuite (AES), but… Need to address other 802.11 security holes such as unauthenticated management frames Cryptographic operations now in the critical path for Fast Handoff ESN reassociated STA cannot access the controlled port of the ESN AP until upper layer authentication completes Authentication of Reassociation-Request/Response, Disassociation required to prevent hijacking

11. November 2001 Tim Moore, Bernard Aboba/Microsoft Questions Should authentication occur before or after reassociation? How do we authenticate management frames? This presentation addresses Reassociation-Request/Response, and Disassociation Notification frames Future work will address authentication of other Management Frames Association-Request/Response, Beacon, Probe-Request/Response, Deauthentication, ATIM

12. November 2001 Tim Moore, Bernard Aboba/Microsoft Alternatives Authentication before reassociation Pros Enables pre-authentication Authentication no longer in the critical path for reassociation Cons If you authenticate management frames, cryptographic operations remain in the critical path (since you need to authenticate the Reassociation Request/Response) If you’re already authenticating reassociation request/response, why do more than “canned” authentication in addition? Reassociation before Authentication Pros Simplicity: authenticate Reassociation-Request/Response, Disassociation, AP issues “canned success” in upper layer authentication if authentication is successful at MAC layer Minimizes cryptographic operations in the critical path for reassociation Cons No pre-authentication

13. November 2001 Tim Moore, Bernard Aboba/Microsoft Proposed Approach Authentication of Reassociate, Disassociate frames Authenticator Information Element added to Reassociation-Request/Response, Disassociation notification frames Authenticator Information Element enables STA and new AP to provide possession of the unicast authentication session key negotiated with the old access point. Support within the Inter-Access Point Protocol (IAPP) New AP passes the Authenticator IE to the with old AP in the Inter-Access Point Protocol (IAPP) Move-Request Old AP validates the Authenticator If successfully validated, old AP sends IAPP Move-Response to new AP Otherwise, old AP silently discards IAPP Move-Request New AP will not send Reassociation-Response STA Reassociation-Request will time out STA, AP will re-authenticate Appropriate 802.11f MIB variable is incremented 802.1X “canned success” sent from AP to STA if Authenticator IE included within the Reassociation-Request is valid.

14. November 2001 Tim Moore, Bernard Aboba/Microsoft 802.11i Fast Handoff

15. November 2001 Tim Moore, Bernard Aboba/Microsoft Authenticator Information Element Assumes that a unicast key is available either for the current AP (Disassociation, Reassociation-Response messages), or with the previous AP (Reassociation-Request message). Authenticator = HMAC-MD5(STA MAC address | AP MAC address | Timestamp, ESSID, key) Timestamp = 8 octet timestamp (see Section 7.3.1.10) to prevent replay The authentication session key derived from the negotiated master key is used (same key as is used to authenticate the EAPOL-Key message)

16. November 2001 Tim Moore, Bernard Aboba/Microsoft Authenticator Information Element 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Element ID | Length | Algorithm | ESSID# | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authenticator +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17. November 2001 Tim Moore, Bernard Aboba/Microsoft Deployability improvements

18. November 2001 Tim Moore, Bernard Aboba/Microsoft The Registration Problem New AP contacts the old AP via IAPP to validate the reauthentication-request, transfer context IAPP runs over IP Implication: New AP needs the IP address of the old AP in order to communicate with it 802.11 enables the STA to obtain the MAC address of the old & new APs Client obtains the MAC address of the old AP when it associates/re-associates with it Client provides the new AP with the MAC address of the old AP in the re-association request But how does the new AP locate the old AP IP address? New AP knows the MAC address of the old AP, not its IP address Need a way to map the MAC address to an IP address

19. November 2001 Tim Moore, Bernard Aboba/Microsoft Alternate Solutions to the Registration Problem Inverse ARP (RFC 2390) Assumes APs are all on the same subnet, so not a general solution Note: Having APs on different subnets does not imply change of subnet for the client (possible for the AP to support dynamic VLANs) AAA protocols Authentication server knows where APs are, but… AAA protocols weren’t designed to solve this problem Registration Service (what’s in 802.11 TgF Draft 2) Problems: Enterprise customers do not wish to deploy yet another service Selecting an AP to run the service requires an election protocol Registration service designs discussed so far (SLPv2, DNS) have serious flaws Include AP IP address(es) in management messages IP address(es) of new AP provided to STA in association-response, reassociation-response STA provides IP address(es) of old AP to new AP in reassociation-request Recommendation: Choice 4 Eliminates need for registration service (and resulting deployment problems)

20. November 2001 Tim Moore, Bernard Aboba/Microsoft Issues with use of SLPv2 for Registration Service SLPv2 designed for use in service discovery, not resolving MAC addresses to IP addresses Use of SLPv2 as a routable version of InverseARP is inefficient SLPv2 requires multicast routing to all access points; not widely deployed SLPv2 limited to use within a single administrative domain – prevents context transfer between domains Inter-domain context transfer should not be prohibited in situations where the trust issues can be worked out For scalability, SLPv2 requires deployment of Directory Agents (DAs) SLPv2 security is inflexible Requires certificate infrastructure Supports only DSA signatures (RSA much more widely used) No known implementations

21. November 2001 Tim Moore, Bernard Aboba/Microsoft Issues with use of DNS for Registration Service DNS not designed as a mechanism for translating MAC addresses to IP Addresses Would require addition of a MAC address record to DNS DNSEXT WG unlikely to agree to this (it’s a bad idea!) DHCPID RR based on a hash of the MAC address DHCPID RR not to be used for alternative purposes Would require APs, DNS servers to support new DNS record types as well as DNS dynamic update DNS dynamic update not yet widely deployed Secure dynamic update implementations not yet interoperable Use by APs would require trust between APs and DNS Server

22. November 2001 Tim Moore, Bernard Aboba/Microsoft Extended Address Information Element Added to Association-Response, Reassociation-Request and Reassociation-Response messages Multiple Extended Address Information Elements can be included if the AP has multiple addresses (IPv4, IPv6, etc.) New AP provides address(es) to STA in Association-Response and Reassociation-Response messages STA provides new AP with address(es) of old AP in the Reassociation-Request message AP uses the address(es) to determine the destination for the Move-Request message sent to the old AP.

23. November 2001 Tim Moore, Bernard Aboba/Microsoft Extended Address Information Element 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Element ID | Length | Type | Address.... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

24. November 2001 Tim Moore, Bernard Aboba/Microsoft New Status Codes Status code Meaning TBD Reassociation-Request denied due to failed authenticator TBD Reassociation-Response denied due to failed authenticator TBD Disassociation denied due to failed authenticator

25. November 2001 Tim Moore, Bernard Aboba/Microsoft Motion To amend the TGi draft to include text detailing usage of the Extended Address and Authenticator Information Elements.

26. November 2001 Tim Moore, Bernard Aboba/Microsoft Feedback?